IMac 27 Inch Early 2009 freezes periodically

Question

For the last couple of weeks my iMac has been freezing and requires a hard reset to fix.  On at least one occasion the screen flashed before it froze.

Answer 1

I have the same problems with my iMac 24" Early 2009 with the Nvidia GeForce GT 120 256MB MXM graphics card. I've done the usual and finally decided that it's a faulty card that only a replacement will fix. Oddly, when I'm home it's more stable when connected via a Displayport DVI adapter to DVI cable to a 24" Dell display. When standalone (like when travelling with my iMac for long stints out of town) it's unusable. It won't run for longer than 30 minutes then I get screen artifacts and freezes and then have to fsck to repair the drive before running it again.
However, I believe that I found the solution. I installed a program which I downloaded after installing XCode called Quartz Debug which has apparently allowed me to disable Quartz Extreme, 2D Acceleration and Quartz GL. I'm not sure which one of these changes has allowed my shaky iMac to run stably now for three hours, including a half hour running a flash game and to play about the same amount of the iTunes download of the movie Avatar, so I'm relatively confident that what I did is working. Both Flash and Quicktime seem to work fine although there does seem to be a bit of screen redraw lag when moving objects on the screen or opening new windows. Other features seem to work fine such as Dock magnification and the Scale and Genie effects.
To disable Quartz Extreme:
Install XCode and then under the XCode menu (the one to the left of "File") click on Open Developer Tool/More Developer Tools...
That will take you to this web page:
https://developer.apple.com/downloads/index.action?name=for%20Xcode%20-
(I don't know if just downloading QD will work without installing XCode, it might)
Select and dowload the package of Applications/Tools called:
Graphics Tools for XCode
Drag the Quartz Debug program to your Applications or Utilities folder. StartQuartz Debug. If you don't get the default tools window open when you start it, look under:
Window/Quartz Debug Settings
. . . and a window will open. You can also hit command-1.
In that menu I made these changes:
Deselected Enable Quartz Extreme
Checked the Disable 2D Acceleration
Checked the Disable radio button under Quartz GL
I am not confident that the settings will persist if I close the program. One website suggested that by forcing quit they will persist, but I'm just not sure. I'll experiment with this later and post again if no one else does. So for now I'm leaving it running and with these settings and no other changes.
I also applied the following changes in Terminal although I'm not sure that it did anything:
http://hints.macworld.com/article.php?story=20120211025246804
So far it's working. I'll follow-up as I have time and once I'm back home.
JoeL
Atlanta, GA